1. Field of the Invention
The present invention relates generally to an apparatus and method for generating laser energy, and more particularly, to passively mode-locked lasers capable of providing ultra-short pulses.
2. State of the Art
Both actively mode-locked lasers and passively mode-locked lasers are well known in the laser art. However, to provide ultra-short pulses for use in environments such as communications applications where ultra-high bit rates are desirable, actively mode-locked lasers are used since typical passively mode-locked lasers have limited operation at low repetition rates.
Passively mode-locked lasers, and in particular, passively mode-locked fiber lasers, have been limited to operation at low repetition rates because of the low gain cross-section associated with rare-earth doped fibers and their tendency to energy loss via cross relaxations at high rare-earth doping levels. This constraint results in typical active fiber lengths being greater than 1 meter for the case of erbium ions. Although useful neodymium fibers can have lengths less than 1 meter, such fiber lengths are uncommon due to impracticalities in manufacture and handling.
A fundamental cavity frequency for a passively mode-locked fiber laser is typically limited to approximately 100 MegaHertz. Although high repetition rates are obtained by inserting subcavities into the cavity of the passively mode-locked fiber laser, this results in the laser becoming sensitive to phase fluctuations between the various cavities and results in unstable operation.
Thus, attention focused on the development of actively mode-locked fiber lasers for achieving higher-harmonic mode-locking using an active modulating scheme. Such active modulating schemes are insensitive to phase fluctuations, and can thereby provide a relatively stable pulse train at repetition rates of, for example, up to several GigaHertz.
Although actively mode-locked lasers have generally been accepted as suitable for use in achieving higher-harmonic mode locking, it would be desirable to provide a passively mode-locked laser which can achieve higher-harmonic mode locking without requiring the active modulation associated with actively-mode-locked lasers. Passive mode-locking techniques avoid the need for modulation electronics, and thus can provide a cost effective, efficient laser.